Lighting is always important because it not only influences human circadian rhythms but also mood and human perception of security and safety. Lighting is also very important for human comfort. Many of us are afraid of the dark. This fear of the dark is usually not fear of darkness itself, but fear of possible or imagined dangers concealed by darkness. When this fear is excessive, it becomes a phobia and humans will often act irrationally and possibly become a danger to themselves and others. In fact, an estimated 8.7% of Americans, or 19.2 million people, suffer from a specific phobia. Also 11% fear the dark and 2% fear storm related events such as deep darkness, thunder and lightning. These fears and phobia's are even more serious in combination when people travel, drive or operate machinery in dreary seasonal or inclement weather conditions.
Lighting is important in all seasonal and weather conditions but especially important during inclement weather which amplifies many human fears. There can be additional impacts if circadian rhythms are negatively altered by external cues such as lighting. One novel way to inexpensively improve both our safety and generate a calming mood during intervals of seasonal or inclement weather is to consider compensating for threatening weather conditions by using lighting to reassure and calm humans in anticipation to and during problematic weather conditions. Also considering weather in conjunction with the time of day for a lighting change can be exploited to minimize circadian disruption or drive a desired change to circadian rhythm in anticipation of a long term weather event such as a major snowstorm. You can pretty much count on lighting being present at locations where humans are active during seasonal weather conditions or inclement weather. Converting the existing lighting to counteract human phobias and fears is a start. Also using the existing lighting infrastructure in combination with weather inputs and other sensors can not only help people sleep better but can pro-actively calm travelers, motorists and pedestrians alike.
But lighting is usually locked into particular solutions that are not flexible, intelligent or easily changed as weather conditions vary. Nor are existing lighting solutions responsive to circadian rhythms. Another ongoing problem with lighting combating the darker side of human nature is the need for proper lighting effects not just whether they can be turned on and off to illuminate an area.
Proper lighting is important. The cones in the human eye are active in higher light-levels (photopic vision) and enable human color vision. Unfortunately, cones are non-functional in low-light. Rods are responsible for our vision in low-light conditions (scotopic vision). Rods are much higher in number than cones (120 million vs. 6 million) and they are more sensitive, but not to color. Within your rods lies Rhodopsin, the chemical that actually allows night vision. When exposed to light, Rhodopsin bleaches and takes around 30 minutes to regenerate. Despite taking so long to regenerate, most night vision begins to return within about 5-10 minutes once the bleaching from bright light stops. Therefore, at night, a row of street lights are effectively flashing and subsequently bleaching Rhodopsin, when seen from a moving vehicle. This is potentially a dangerous situation since the effect will reduce vision at night (while driving). A moonless and starless cloudy night only exacerbates the situation. Add in weather events such as fog, rain or snow and a much bigger problem emerges due to light reflection, refraction and scattering obscuring vision.
Therefore, it is important to manage lighting during transitions from light to darkness to avoid temporarily inhibited vision. The Rhodopsin contained within the rods in your eyes is less sensitive to the color red (longer wavelengths of light). You may have seen the Military using red light to allow them to read maps while preserving their night vision. However, there has been a shift recently to using green light or a blue-green light instead of red. Green allows for more visual acuity and better differentiation between colors at low-light levels. Both red and green light at high output will still disable your night vision, no matter what color, so intensity is still part of the lighting management challenge. Therefore, choose your colors carefully and avoid excessive illumination at night. It is also important to soften any change since it's better than quickly getting hit with a dark environment or alternating light/dark environments (flashing lighting . . . etc.). If you're doing anything at night, high-output white light will disable your night vision fastest. But, you can always benefit from a different spectrum and avoiding abrupt changes, provided it's reasonably low-light.
Today the consensus is that a dim red light will have the least effect on your night vision. That is why astronomers and soldiers use flashlights with red lenses. This means it is important to control not only light color but intensity. It has been found that animals (hamsters in one study) exposed to red light at night had significantly less evidence of depressive-like symptoms and changes in the brain linked to depression, compared to those that experienced blue or white light. Research findings suggest that if we could use red light when appropriate for night-shift workers, it may reduce some of the negative effects on their health from white light. This also applies to periods of darkness caused by seasonal (in some parts of the world such as Seattle) or inclement weather. Seasonal and inclement weather is known to impact humans by disrupting circadian rhythms which can be counteracted by modulating external cues such as lighting.
Our eyes sense light slightly differently in darkness than in daylight. In day light our eyes are most sensitive to green light, they are slightly less sensitive to yellow and blue light, they are only one half as sensitive to orange and lavender light and they are only one tenth as sensitive to red and violet light. Thus for viewing at a distance, a 100 watt green light bulb can be seen about 3 times farther than a red or violet bulb of the same light power. Thus red is the worst color for seeing at a distance. You can observe this on a clear dark night from a hilltop looking at distant traffic signals. The yellow light will be brightest, the green light (which is slightly blue) will be seen almost as bright and the red light will be quite dim. For the same reason, on newer cars with yellow turn indicators, you will notice that they are much brighter than the red turn indicators on older cars.
In darkness the eyes maximum sensitivity shifts toward yellow-green wavelengths with the other colors staying at about the same sensitivity. As we understand more and more about human anatomy and psychology we can apply that knowledge to utilizing lighting for positive impacts to health, safety and human behavior.
The proper mix of color, intensity and spectral range is useful in enhancing human vision in many lighting situations, especially in inclement weather. But both indoor and outdoor lighting solutions today do not consider weather or circadian rhythms separately or jointly during their operation and have no ability to react to actual and anticipated weather conditions.
Increasingly, new applications beyond simply providing the ability to schedule your lights to turn on and off at specific times throughout the day will emerge and allow the lighting industry to take the next step of dynamically addressing all our lighting needs; at home, office, shopping etc., while traveling between these activities in any kind of season or weather conditions. In addition, impacts to human circadian rhythms can be modulated as needed in any kind of weather or seasonal conditions. This evolutionary step enables cities to become increasingly responsive by adjusting lighting to a variety of real-time weather conditions to enhance safety, increase human comfort, set moods, improve livability and maximize the quality of life.